Review note: The PA2V2 headphone amp is a high-level amplifier that's designed to be connected to the headphone jack of portable music players, cellphones, and even laptop or desktop computers should a user require additional amplification from their computer. In my case, and since the Apple i-devices have a true Line Out (Line Out Dock, or 'LOD') that bypasses the i-device volume control circuit, I run the PA2V2 from the LOD using a short FiiO LOD cable.

Setting the volume for use with the i-device LOD is simple - the only option is to use the PA2V2 volume control knob, which by the way is the most ideal method of controlling volume for music playback. When using this amp from the music player's headphone jack, there will be 2 volume controls inline - the music player's volume control and the PA2V2 volume control. In that case I recommend finding a position with the music player volume control that produces clean undistorted sound with full dynamics - not too low and not too high - and then control the volume with the PA2V2 only. You could do the reverse of course, but that depends on your particular situation and what might be more convenient for you. My experience with the PA2V2 has given me more than ten times the volume I need to play all of the above noted headphones to ear-splitting levels, which tells me that clipping or other dynamics limitations are not going to be an issue with this amp.

The PA2V2 comes with two cables - one for charging the amp from a standard USB jack, and the other a short double-ended miniplug (3.5 mm) cable for connecting to music players' headphone jacks. The PA2V2 contains 2 AA NiMH rechargeable batteries, and the no-less-than-spectacular news there is that those batteries are available everywhere for a low price, and the PA2V2 runs about 100 hours on a single charge. The PA2V2 is 2.6 inches square and 1.1 inches high, with an additional protrusion of .625 inches for the volume control knob. The case is a strong ABS-type plastic, so I wouldn't expect any durability issues, although the volume control knob and on-off switch should be protected. There is no standard carry case for this amp, but I'm currently using an old 'Timbuk2' soft case made for the iPod classic models. That case can be carried horizontally on a belt, or more ideally for my use - carried vertically on the strap of any shoulder bag, owing to the wraparound velcro design.

The other amps that I have on hand to compare to the PA2V2 are the FiiO E07k, the Decware Zen Head, and the Beyerdynamic A200p. The Zen Head ($375 USD), like the PA2V2, is strictly a headphone amp, requiring the music player's internal DAC to convert the digital music tracks to analog format. The FiiO E07k is a headphone amp in the same sense for use with i-devices and most other portable music players, but can also be used as a computer USB DAC plus headphone amp (not applicable for this review). The Beyer A200p ($300 USD) is an i-device-compatible DAC and headphone amp, so it has the (expensive) advantage of having its own high-quality internal DAC, which bypasses the internal DAC contained in each of the Apple i-devices.

Compared to the A200p, the PA2V2 didn't have as much resolution of upper harmonic detail, but I think the PA2V2 does have detail comparable to the A200p's internal headphone amp - the difference being the PA2V2 depends on the i-device DAC which has less resolution than the A200p's internal DAC. Compared to the FiiO E07k (both analog amps in this case), the PA2V2 has a better sense of detail and "air", which by the way does not derive from signal processing such as the 'Crossfeed' DSP's used in some headphone amps. Comparing to the $375 Zen Head, I expected the Zen Head to do better, since according to its designer it eschews circuits that can reduce signal quality - reverse-polarity protection for example. My best-case track for this test was David Hazeltine's "Fur Elise" from HDTracks. Although my i-devices are unmodified and can play only 44-48 khz WAV tracks at best, there's enough detail above ~15 khz or so from the internal DAC to reproduce the fine harmonics and decay necessary to compare these amps.

In the final analysis, whether using the PA2V2 as a headphone amp attached to a cellphone or comparable portable music player, or as the final component in a music system that feeds the PA2V2 with an analog signal suitable for headphones (usually the output of a USB DAC), I'd judge the PA2V2 to be as good (or about as good) as any headphone amp up to the Zen Head's quality level, and the Zen Head is a very well-made headphone amp. The only uncertainty in that comparison is due to the resolution limit of the i-device DAC, since I don't have a stand-alone DAC to drive the PA2V2 or Zen Head amps, to see how those combinations would compare to the Beyerdynamic A200p, or to my Microstreamer computer DAC plus headphone amp.

Looking at the PA2V2 from the front, there's the volume control knob, the on-off rocker switch, the small (less than 3.5 mm) input for USB charging, the 3.5 mm analog input jack, and the 3.5 mm headphone out jack. Between the volume knob and the on-off switch is a red LED that serves as the power ON indicator. There are no other features on the outside of the case, so slipping the PA2V2 into nearly any suitable carrying case will have all necessary controls facing up (or forward) and totally accessible to the user.

The following music tracks were my primary test material for this review:

Chromatics - I'm On Fire (Synth-Pop, female lead): This music track has a very spacious sound, providing a pleasant stereo image. The voice and instruments are well balanced and the tambourine sounds very realistic, undoubtedly because of the excellent harmonic reproduction.

David Hazeltine - Fur Elise (Jazz): A very high-quality recording from HDTracks, where the instruments are reproduced smoothly with a spacious ambiance. The wire-brush-on-cymbal harmonics are very extended and detailed with the PA2V2.

Jimmy Smith - Basin Street Blues (early 60's): This track has several loud crescendos of brass and other instruments that don't sound clear and detailed with some amps. Listen particularly to the second crescendo at 15 seconds in for maximum detail effect. I'd like to emphasize that these crescendos are probably the worst-case test I have for instrumental separation and detail, and the PA2V2 aces them.

Kellogg Auditorium, Battle Creek Michigan, Aeolian-Skinner Organ - Pedal, 32', Resultant, Arpeggio: This 16 hz organ pedal tone differs from other tones in that you won't "hear" it - you only feel it. Although most music tones have harmonics, the harmonics from this tone are too weak to provide any "feel", so what you actually hear is not part of the fundamental 16 hz tone. There are ~30 hz sounds in the outdoor environment in big cities, generated by large trucks, buses, and subway trains, and they have a quality of "rumble" that's similar to some deep-bass tones found in music. This 16 hz tone is easily distinguished from those sounds when compared using gear that's distortion-free with suitable dynamic overhead at 16 hz. With this amp you can almost count the 16 "beats" per second in this track, since even with harmonics of 32 hz or above, the 16 hz tone is so clearly reproduced in the "feel" sense that the only reason I can't count the beats (peak and trough cycles) is because they're too close together.

Porcupine Tree - Trains (Pop-Rock): This track opens with some nicely-detailed string sounds and a forward-sounding male voice with a higher-than-average register. There are a series of "clip-clop" effects starting at 3:19 that should sound like they were made with wooden blocks of some kind. The PA2V2 reproduces these sounds faithfully.

Richard Strauss (Mester-Pasadena) - Also Sprach Zarathustra (opening) (Classical): The granddaddy of bass is in the opening 1:50 of this recording, and I've heard it only once on a large and expensive loudspeaker system in Cleveland. For most people, that experience would be indistinguishable from being in a fairly strong earthquake. The PA2V2 reproduces that experience as well as any headphone amp that I've heard.

Trombone Shorty - Backatown (Jazz-Funk): The deep bass impacts here are quite strong, and work very well with the horns and other instruments. The PA2V2 delivers the impacts with great weight and detail, and the horns have the kind of bite that gives them a wonderfully realistic sound.

William Orbit - Optical Illusion (Billy Buttons Mix) (Electronic): The string tones beginning at 0:18 are fairly soft but detailed, and while the bass isn't very deep, it adds a solid underpinning to the music. This is a good track for comparing amps because many of the tones and their harmonics are soft and fairly subtle, and with some of those amps the harmonics won't reproduce properly.